Unexpected clang build warning

main.c

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <alloca.h>

void eval_program(char *program) {
    int program_len = strlen(program);
    int data_index = 0, instruction_index = 0;

    char c;
    while (instruction_index < program_len) {
        c = *(program + instruction_index);

        switch (c) {
        case '>':
            data_index++;
            instruction_index++;
            break;
        default:
            instruction_index++;
            break;
        }
    }
}

void *realloc_or_die(void *ptr, size_t size) {
    void *p = realloc(ptr, size);

    if (p == NULL) {
        fprintf(stderr, "Out of memory! Exiting.");
        exit(1);
    }

    return p;
}

char *read_string(int file_descriptor) {
    char *s = NULL;
    int total_bytes_read = 0;

    int BUFFER_SIZE = sizeof(char) * 1024;
    char *temp_buffer = alloca(BUFFER_SIZE);

    int bytes_read;
    while ((bytes_read = read(file_descriptor, temp_buffer, BUFFER_SIZE))) {
        if (bytes_read == -1) {
            fprintf(stderr,
                    "Could not read from file descriptor %d, exiting.\n",
                    file_descriptor);
            exit(1);
        }

        s = realloc_or_die(s, total_bytes_read + bytes_read);
        memcpy(s + total_bytes_read, temp_buffer, bytes_read);
        total_bytes_read += bytes_read;
    }

    s = realloc_or_die(s, total_bytes_read + 1);
    s[total_bytes_read] = '\0';

    return s;
}

int main() {
    char *program = read_string(0); // read from stdin
    eval_program(program);
    free(program);

    return 0;
}

output.txt

$ scan-build gcc -Wall -g -std=c99 main.c 
scan-build: Using '/usr/bin/clang' for static analysis
main.c:13:11: warning: Assigned value is garbage or undefined
        c = *(program + instruction_index);
          ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
scan-build: 1 bug found.
scan-build: Run 'scan-view /tmp/scan-build-2015-01-21-232411-5285-1' to examine bug reports.