soc: qcom: mdt_loader: Deal with zero e_shentsize

Firmware that doesn't provide section headers leave both e_shentsize and
e_shnum 0, which obvious isn't compatible with the newly introduced
stricter checks.

Make the section-related checks conditional on either of these values
being non-zero.

Fixes: 9f9967fed9d0 ("soc: qcom: mdt_loader: Ensure we don't read past the ELF header")
Reported-by: Val Packett <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Reported-by: Neil Armstrong <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Bjorn Andersson <[e-mail]>
Fixes: 9f35ab0e53cc ("soc: qcom: mdt_loader: Fix error return values in mdt_header_valid()")
Tested-by: Neil Armstrong <[e-mail]> # on SM8650-QRD
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/soc/qcom/mdt_loader.c

···
 	if (phend > fw->size)
 		return false;
 
-	if (ehdr->e_shentsize != sizeof(struct elf32_shdr))
-		return false;
+	if (ehdr->e_shentsize || ehdr->e_shnum) {
+		if (ehdr->e_shentsize != sizeof(struct elf32_shdr))
+			return false;
 
-	shend = size_add(size_mul(sizeof(struct elf32_shdr), ehdr->e_shnum), ehdr->e_shoff);
-	if (shend > fw->size)
-		return false;
+		shend = size_add(size_mul(sizeof(struct elf32_shdr), ehdr->e_shnum), ehdr->e_shoff);
+		if (shend > fw->size)
+			return false;
+	}
 
 	return true;
 }